Project Summary Richter's transformation (RT) is an aggressive and incurable diffuse large B cell lymphoma (DLBCL) that clonally evolves from chronic lymphocytic leukemia (CLL), the most prevalent leukemia in adults. Patients who develop RT have few treatment options and face a grim prognosis of only 6-8 months despite aggressive multimodal therapy. Importantly, RT has become the most common type of disease progression observed in CLL patients receiving targeted therapies such as the BTK inhibitor Ibrutinib and the Bcl-2 inhibitor Venetoclax (ABT-199). As the use of these agents continues to grow, emergence of resistance and progression to RT are of increasing clinical concern. Recent data suggests that RT is a biologically and clinically distinct disease entity from CLL and DLBCL. Because of this, well-defined risk factors for CLL cannot be applied to predict which patients will develop RT, indicating clear, unmet needs to identify epigenomic lesions predictive of patients at the highest risk for transformation and to bring forward novel treatment options with real curative potential for this fatal complication. Clonal evolution is considered a key feature of cancer progression and relapse. About 80-90% of RT cases arise from the underlying CLL clone although the mechanisms driving RT are poorly understood. Currently no driver mutations have been identified leading to the hypothesis that acquired epigenetic lesions may favor the emergence of the more aggressive RT clone. Approximately 50% of RT tumors display epigenetic changes affecting cMYC over-expression and P53 inactivation, suggesting these pathways may play a major role in the pathogenesis of RT. BRD4 and PRMT5 are epigenetic modifiers essential for P53 and c-MYC activity that we have shown to have transforming potential in several lymphoma models. In contrast to CLL patients who do not develop RT, our preliminary data show that PRMT5 is abundantly over-expressed in CLL tumor cells months to years prior the development of RT. Novel targets of both PRMT5 and BRD4 regulation have not been characterized in CLL/RT. Our preliminary data supports the existence of a BRD4-PRMT5-MYC/P53 feed- forward loop that drives the malignant phenotype of RT and represents an ideal driver axis to deliver targeted therapy. This proposal will utilize a highly novel, integrated epigenomic approach to mechanistically address how BRD4 and PRMT5 contribute toward global epigenetic changes favoring the emergence of the RT clone. If successful our experiments will identify unique epigenomic disease associated with risk of RT. Additionally, the use of innovative, spontaneous, immune competent murine models of RT we have generated coupled with novel, first-in-class agents that selectively target BRD4 and PRMT5 place us in a unique position to make a significant impact for patients with this disease by developing preventive and therapeutic approaches that can be translated into the clinic.